68 DR KNOX on the Comparative Anatomy of the Eye. 



the bird, and forms, as it were, the intermediate link, by which 

 the classes Mammalia and Aves are, so far as regards vision, 

 united together. I have already explained, why, in the deer, 

 though the mode by which the optic nerve enters the eye be the 

 same as in birds, there is still no distinct marsupium formed, 

 because the chief vessels do not pass through the vitreous hu- 

 mour, but accompany the retina. As the optic nerve passes 

 through the sclerotic in birds, this latter membrane forms, as it 

 were, a lengthened sheath (by a separation of its tunics) in which 

 the nerve proceeds for some way, before it penetrates into the 

 interior of the eye. In consequence of this peculiar form of the 

 sclerotic, the termination of the nerve which regards the vitre- 

 ous humour, is bounded by two strong, tendinous, straight lines ; 

 these are, of course, formed by the edges of the sclerotic. The 

 same distribution of the sclerotic and nerve takes place in the 

 deer, and extends even to the passage of the vessels, from the 

 exterior to the interior parts of the eye, mid-way between the 

 optic nerve and cornea. At these points (four or five in num- 

 ber), the choroid, by the transmission of prolongations, evidently 

 communicates with the exterior of the eye-ball *. The vessels 

 do not pass directly through the sclerotic, but between its layers ; 

 one edge of the sclerotic overhangs the other, and the more pro- 

 jecting line is strong, tendinous, and fixed down to the inner 

 surface of the sclerotic. But it is not in these points only that 

 the strong resemblance of the deer's eye to that of the bird holds 

 good ; the great ciliary muscle is remarkably broad and distinct, 



* In the number of the Philosophical Transactions for 1810, (I quote from me- 

 mory), four distinct muscles are described within the sclerotic, in the eye of the rhi- 

 noceros. The situation of these muscles corresponds exactly with the prolongation of 

 the choroid membrane described in the text. I have examined the eye of the Afri- 

 can or two-horned rhinoceros, but do not remember to have observed any such 

 cles in it. 



